1. Field of the Invention
The present invention relates to an actuator using a shape memory alloy (SMA), and more particularly, to an SMA actuator which has a high reaction speed and remarkably reduced power consumption.
2. Description of the Related Art
A shape memory alloy (SMA) actuator may be classified into a so-called ‘passive actuator’ which operates according to a change in ambient temperature and a so-called ‘active actuator’ which uses phase transformation caused by ohmic heat through flow of current.
The passive SMA actuator mostly shows a behavior proportional to ambient temperature, and the active SMA actuator mostly operates in an ON/OFF-type fashion. In particular, because of many advantages, such as lightweight, low cost, and a noise-free feature, the active SMA actuator is used in place of a solenoid, a motor, and so forth in various fields such as vehicles, cameras, electric home appliances, etc.
FIGS. 1 and 2 show an example of a conventional active SMA actuator 1. The conventional active SMA actuator 1 includes an SMA element 2 which is in the form of a wire and has one end portion that is positionally fixed, a movable member 4 which couples to the other end portion of the SMA element 2 and is positionally movable between a first position and a second position, and a counterpart spring 3 which couples to the movable member 4 and elastically biases the SMA element 2 in a direction the SMA element 2 extends.
Thus, if electric current flows in the SMA element 2 according to a signal of a controller (not shown), and thus ohmic heat is generated, then the SMA element 2 contracts to its original state and the movable member 4 is located at the first position as shown in FIG. 1.
Since the SMA element 2 is in a high-temperature phase (austenite phase) at a high temperature, a contractile force generated by the SMA element 2 is larger than that of the counterpart spring 3, such that the SMA element 2 contracts and the counterpart spring 3 extends, as shown in FIG. 1. In this state, the power has to be continuously supplied to fix the movable member 4 at the first position.
If the electric current in the SMA element 2 is short-circuited according to a signal of the controller and thus the ohmic heat is removed, the temperature decreases. Since the SMA element 2 is in a low-temperature phase (martensitic phase) at a low temperature, the contractile force capable of resisting extension of the SMA element 2 is smaller than that of the counterpart spring 3 and thus the SMA element 2 is likely to be deformed, such that the counterpart spring 3 contracts, the SMA element 2 extends, and the movable member 4 positionally moves to the second position. If a wire diameter of the counterpart spring is 0.3 mm, it takes about 9-10 seconds to cool the counterpart spring 3 from 90° C. to room temperature of 20° C., such that about 10 seconds are required for the movable member 4 to move from the first position to the second position.
Generally, the conventional active SMA actuator 1 is used in an ON/OFF operating manner, and does not operate in a proportional fashion like a passive actuator. In the conventional active SMA actuator 1, an SMA element in the form of a coil spring capable of generating a tensile force or a compressive force may be used in place of the SMA element 2 in the form of a wire. The counterpart spring 3 may also be any other spring capable of generating a compressive force.
However, the conventional active SMA actuator 1 has a limited application range because of several problems.
A first problem is power. As shown in FIG. 1, to maintain the SMA element 2 in a contracted state, the high-temperature phase has to be maintained by continuously supplying power to the SMA element 2 (ON state), resulting in high power consumption. If the power supplied to the SMA element 2 is cut off (OFF state), the state unintentionally returns to the state shown in FIG. 2, such that due to this power problem, the application of the conventional active SMA actuator 1 to a cellular phone, a camera, etc., which uses a compact battery is limited.
A second problem is heating speed and cooling speed. The SMA element 2 needs to be rapidly heated and maintained at a proper constant temperature, but it is difficult to determine proper current to be supplied. If the current is too high, the SMA element 2 may be heated rapidly, but it may also be overheated due to continuous heating; whereas if the current is too low, much time is required to heat the SAM element 2 in spite of avoiding to overheat the SMA element 2.
If current supply is stopped according to the signal of the controller as shown in FIG. 2, the SMA element 2 should be rapidly cooled and immediately enter the low-temperature phase (martensitic phase), but a relatively long time is required to cool the SMA element 2 because the SMA element 2 is cooled in an air convection manner. This problem becomes worse especially when the diameter of the wire-type SMA element 2 is large or the SMA element 2 is installed in a closed space.
As a result, the conventional active SMA actuator 1 has much power consumption because power has to be continuously supplied in the high-temperature phase (austenite phase), and has a low reaction speed because it takes about 10 seconds to cool the conventional active SMA actuator 1 from the high-temperature phase to the low-temperature phase due to the stop of current supply.